In recent years, reforestation has become important and many lumbering concerns plant new tree seedlings after having harvested the forest. Tree seedings are started in nurseries and then transplanted when they reach a certain height. It has been found that trees which have been transplanted from the nursery to the field often lack the root stability of trees which have germinated and grown naturally. In particular, lodge pole pines which have been started in a tree nursery in man-made containers once transplanted, have toppled.
One problem in container grown seedlings is root spiralling. This occurs because the walls of the cavities constrict the tree's growth and produce root bound seedlings.
A typical tree seedling has both a tap root which goes vertically downwards and also lateral roots which generally grow horizontally. When the horizontal laterals reach the cavity walls, they begin to spiral about the tap root and consequently the tree commences its life after being transplanted with an unstable root system. To overcome this problem, downwardly extending ribs are molded into the inner cavity walls of the seedling container cavities. When the lateral roots hit the rib, they are directed downwards and roots spiralling is prevented. When the roots reach the bottom of the cavity which is open to the air, their growth is temporarily halted by air pruning.
This recent method has been found to produce superior root systems on tree seedlings but still has several disadvantages. In particular, because all of the lateral roots are directed vertically downwards around the tap root, tree seedlings once transplanted tend to suffer rotation and can topple in a strong wind. This is due to the lack of horizontal lateral roots.